The present invention relates to a device for transmitting motion between the rotor of a synchronous permanent-magnet motor and the working part having an increased free rotation angle.
It is known that electric motors with permanent-magnet rotor have a structural layout which includes a stator, with an electromagnet constituted by a lamination pack and by associated windings, and a rotor which is arranged between two poles formed by the stator and is axially crossed by a shaft which is rotatably connected to a supporting structure.
It is also well-known that the higher the inertia of the load applied to a synchronous motor, the more difficult it is to start said motor.
Starting in fact occurs as a transient process in which the direction of rotation, the speed and the current change until synchronous operation is achieved.
During this transient process the rotor oscillates due to the alternating magnetic field produced by the stator, which by inducing a torque on the permanent-magnet rotor tends to move said rotor into a position in which the magnetic field of said rotor is aligned with the stator field.
If, during this hunting, the rotor acquires enough kinetic energy to move slightly out of its alignment position, it undergoes a further acceleration which makes it turn slightly further, and so forth, until synchronous operation is achieved.
For an equal power level, the extent of the oscillations produced in the rotor increases as the inertia of the applied load decreases; accordingly, the rotor is able to accelerate, acquiring a speed which allows it to synchronize with the alternating field of the stator.
If instead the inertia of the load is significant, the extent of the oscillation of the rotor is limited and synchronous operation cannot be achieved.
As the inertia of the load increases, the extreme situation occurs in which after power has been supplied to the stator the rotor cannot even begin its oscillation, i.e., it remains motionless in its equilibrium position.
When the inertia of the load is not too high with respect to the power of the motor (for example the impeller of a centrifugal pump), couplings of the mechanical type are currently widely used; said couplings are inserted between the load and the rotor and allow said rotor to oscillate freely, during starting, through a certain rotational angle.
This is the case of so-called toothed couplings, in which a first driving tooth is eccentric with respect to the rotation axis and is rigidly coupled to the rotor, while a second driven tooth is also eccentric with respect to the rotation axis and is rigidly coupled to the load.
In this manner, during the starting transient the rotor is disengaged from the inertia of the load and this makes it easier to achieve synchronous operation.
Accordingly, there is a free rotation through a certain angle (usually 180 sexagesimal degrees) followed by impact when the load is engaged, providing a direct connection between the load and the rotor, which are in practice rigidly coupled during operation.
Therefore the free rotation transient allows the motor to reach the synchronous state and to develop a torque which allows it to overcome the starting moment of inertia of the load.
After this starting transient, the torque, and therefore the power, required in the steady state is very often far lower than the static torque.
However, there are applications in which the moment of inertia of the load is so high (for example the impeller of a centrifugal pump used as a washing pump in dishwashers) that even the above mentioned couplings are unable to start it unless the motor is oversized to the point of being excessively expensive to manufacture and use and is therefore of no interest for the user.
As the inertia and resisting torques increase, the generated static torque must in fact also increase, with the obvious limits posed by the maximum stator flux allowed by permanent magnets, on penalty of demagnetizing them, and by the ability of the active components (iron and copper) to dissipate the temperatures generated due to the high circulating currents that occur even after the transient starting step has ended.
A further consequence is the high level of vibration generated due to the angular torque oscillations caused by a disproportionate choice of motor size in order to be able to produce the torque required for starting.
The effect of these oscillations, which are produced at every turn of the rotor, is to produce an instantaneous separation of the two teeth of the coupling, consequently generating noise.
The high static torque also makes it difficult to provide appropriate dimensions for the coupling owing to the intense stresses produced during impact and leads to the generation of excessive noise.
In such cases, it is thought that one solution for the initial driving of the load might be, apart from oversizing the motor, to increase the angle of free rotation of the rotor with respect to the load, i.e., to provide a greater uncoupling of the motor from the load during the starting transient.
This is currently constrained by the materials used for the parts of the coupling, particularly the teeth, which are usually made of plastics, as well as by the radial dimensions of the rotor, which are necessarily modest (on the order of a few tens of millimeters), bearing in mind that the impact of one tooth against the other during starting is considerable.
The interposition of shock-absorbing means, which is sometimes provided, worsens the situation because said means also require their own angular extension and accordingly their presence further reduces the available free rotation angle.
It is also known that synchronous permanent-magnet motors are bidirectional; i.e., at power-on the rotor can equally start turning clockwise or counterclockwise.
While this is not a problem in the case of the actuation of centrifugal pumps with radial vanes, it is a considerable limitation for centrifugal pumps which have a particular vane configuration and accordingly a single direction of rotation for the impeller.